A water-absorbing resin (SAP/Super Absorbent Polymer) is a water-swellable, water-insoluble polymer gelatinizer, and is frequently used for absorbents. Examples of the absorbents encompass sanitary materials such as disposable diapers and sanitary napkins, agricultural and horticultural water retaining agents, and industrial waterproofing materials.
Examples of a raw material of the water-absorbing resin encompass a large number of monomers and hydrophilic polymers. Industrially most common among such water-absorbing resins is a polyacrylic acid (salt)-based water-absorbing resin in which an acrylic acid and/or a salt thereof is used as a monomer(s), because such a polyacrylic acid (salt)-based water-absorbing resin has high water absorption performance.
The water-absorbing resin is produced as a product in the form of particles through various processes such as polymerization, drying, crushing, classification, and surface crosslinking (Non-Patent Literature 1). In line with enhancing performance of disposable diapers for which the water-absorbing resin is mainly used, the water-absorbing resin is required to have a large number of functions (physical properties). Examples of the functions (physical properties) in addition to fluid retention capacity encompass gel strength, water soluble component, water absorption speed, fluid retention capacity under pressure, liquid permeability, particle size distribution, urine resistance, antibacterial property, impact resistance (damage resistance), powder fluidity, deodorizing property, coloration resistance (whiteness), and dust resistance.
In addition, for the purpose of increasing productivity (particularly polymerizability) and water absorption performance of the water-absorbing resin, a technique to add an additive to a monomer has been proposed.
Specifically, having been proposed are techniques concerning an internal crosslinking agent as an additive, that is, a technique to disperse an internal crosslinking agent using a dispersing agent (Patent Literature 1), a technique to use internal crosslinking agents having different reaction mechanisms in combination (Patent Literatures 2 and 3), and a technique, in which a length of time between addition of an internal crosslinking agent and introduction of a resultant mixture into a polymerization device is specified (Patent Literature 4).
Also having been proposed are techniques concerning a polymerization initiator as an additive, that is, a techniques concerning a photopolymerization initiator having a benzoyl group (Patent Literature 5), a technique in which a polymerization initiator is continuously added (Patent Literature 6), and a technique in which a polymerization initiator is diluted and then added (Patent Literature 7).
Also having been proposed are techniques concerning a reducing agent as an additive, that is, a technique in which a sulfinic acid derivative is added and polymerized (Patent Literature 8), a technique in which an oxygen concentration is reduced with the use of a reducing agent and then polymerization is carried out (Patent Literature 9), and a technique in which a reducing agent is added in a polymerization device (Patent Literature 10).
Also having been proposed are techniques concerning a chain transfer agent as an additive that is a technique in which a chain transfer agent is used in carrying out reverse phase suspension polymerization (Patent Literature 11), techniques in each of which a hypophosphorous acid is used (Patent Literatures 12 and 13), and a technique in which a thiol compound is used (Patent Literature 14).
Also having been proposed are techniques concerning a surfactant as an additive, that is a technique in which a fluorine-based surfactant is used (Patent Literature 15), a technique in which an organometallic surfactant is used (Patent Literature 16), and a technique in which solubility of gas is lowered in the presence of a surfactant (Patent Literature 17).
Also having been proposed are techniques concerning a chelating agent as an additive, that is a technique in which polymerization is carried out in the presence of a certain chelating agent (Patent Literatures 18 and 19) and a technique in which a ratio of iron ion to a chelating agent is controlled (Patent Literature 20).
Also having been proposed are techniques concerning, as additives, a water-insoluble polymer (particularly a water-absorbing resin fine powder) and a fibrous material, that is techniques in each of which a water-absorbing resin fine powder is added to an aqueous monomer solution so as to polymerize the aqueous monomer solution (Patent Literatures 21 through 25), a technique in which starch is added to an aqueous monomer solution so as to polymerize the aqueous monomer solution (Patent Literature 26), and a technique in which a fibrous material is added to an aqueous monomer solution so as to polymerize the aqueous monomer solution (Patent Literature 27).
Also having been proposed are techniques concerning inorganic fine particles as an additive, that is a technique in which an aqueous monomer solution containing a water-insoluble solid matter is subjected to high-temperature polymerization (Patent Literature 21) and a technique in which inorganic fine particles are added to carry out polymerization (Patent Literature 28).
Also having been proposed are techniques concerning a foaming agent (gas generating agent) as an additive, that is a technique in which a carbonate is used (Patent Literature 29), a technique in which an encapsulated foaming agent is used (Patent Literature 30), a technique in which a reverse phase suspension polymerization is carried out in the presence of a foaming agent (Patent Literature 31), techniques in each which an azo compound is used (Patent Literatures 32 and 33), and a technique in which a solid azo compound is dispersed in an aqueous monomer solution (Patent Literature 34).
Also having been proposed are techniques for improving a water absorption speed of a water-absorbing resin, that is techniques concerning methods for continuously mixing gases (Patent Literatures 35 and 36), a technique in which gas bubbles are dispersed in the presence of a surfactant and a solubilizer (Patent Literature 37), a technique in which miorobubbles are introduced (Patent Literature 38), a technique in which gases are mixed with the use of a microreactor (Patent Literature 39), and a technique in which gas is dissolved in an aqueous monomer solution under reduced pressure (Patent Literature 40).